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Scalar (V/f) Control of 3-Phase
Induction Motors
Authors: Bilal Akin, Nishant Garg
Texas Instruments, Inc. C2000 Systems and Applications
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Contents
Introduction ................................................................................................................................................3 Induction Motors.........................................................................................................................................3 Scalar Control ............................................................................................................................................5 Benefits of 32-bit C2000 Controllers for Digital Motor Control ................................................................. .8 TI Motor Control Literature and DMC Library ............................................................................................9 System Overview .................................................................................................................................... 10 Hardware Configuration .......................................................................................................................... 13 Software Setup Instructions to Run HVACI_Scalar Project .................................................................... 16 Incremental System Build ....................................................................................................................... 17
Abstract This application note presents a solution to control an AC induction motor using the TMS320F2803x microcontrollers. TMS320F2803x devices are part of the family of C2000 microcontrollers which enable cost-effective design of intelligent controllers for three phase motors by reducing the system components and increase efficiency. In this system, the scalar control (V/Hz) of induction motor will be experimented with and will explore the use of speed control. The user can quickly start evaluating the performance of V/Hz system with this method.
This application note covers the following:
A theoretical background on scalar motor control principle. Incremental build levels based on modular software blocks. Experimental results
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Introduction The motor control industry is a strong, aggressive sector. To remain competitive new products must address several design constraints including cost reduction, power consumption reduction, power factor correction, and reduced EMI radiation. In order to meet these challenges advanced control algorithms are necessary. Embedded control technology allows both a high level of performance and system cost reduction to be achieved. According to market analysis, the majority of industrial motor applications use AC induction motors. The reasons for this are higher robustness, higher reliability, lower prices and higher efficiency (up to 80%) on comparison with other motor types. However, the use of induction motors is challenging because of its complex mathematical model, its non linear behavior during saturation and the electrical parameter oscillation which depends on the physical influence of the temperature. These factors make the control of induction motor complex and call for use of a high performance control algorithms such as “vector control” and a powerful microcontroller to execute this algorithm. Scalar control is the term used to describe a simpler form of motor control, using non-vector controlled drive schemes. An ACI motor can be led to steady state by simple voltage fed, current controlled, or speed controlled schemes. The scalar variable can be manipulated after obtaining its value either by direct measurement or calculation, and can be used in both open loop and closed loop feedback formats. Although its transient behavior is not ideal, a scalar system leads to a satisfactory steady state response. Induction Motors Induction motors derive their name from the way the rotor magnetic field is created. The rotating stator magnetic field induces currents in the short circuited rotor. These currents produce the rotor magnetic field, which interacts with the stator magnetic field, and produces torque, which is the useful mechanical output of the machine. The three phase squirrel cage AC induction motor is the most widely used motor. The bars forming the conductors along the rotor axis are connected by a thick metal ring at the ends, resulting in a short circuit as shown in Fig.1. The sinusoidal stator phase currents fed in the stator coils create a magnetic field rotating at the speed of the stator frequency (ωs). The changing field induces a current in the cage conductors, which results in the creation of a second magnetic field around the rotor wires. As a consequence of the forces created by the interaction of these two fields, the rotor experiences a torque and starts rotating in the direction of the stator field. As the rotor begins to speed up and approach the synchronous speed of the stator magnetic field, the relative speed between the rotor and the stator flux decreases, decreasing the induced voltage in the stator and reducing the energy converted to torque. This causes the torque production to drop off, and the motor will reach a steady state at a point where the load torque is matched with the motor torque. This point is an equilibrium reached depending on the instantaneous loading of the motor. In brief:
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Rotor flux A’
A
B
C
C’
B’
ia
Rotor rotation
Stator flux
Ω
Ω=Ω .sR
SΩ
Aluminum bar
Figure 2: Squirrel cage rotor AC induction motor cutaway view
Figure 1: Induction Motor Rotor • Owing to the fact that the induction mechanism needs a relative difference between the motor
speed and the stator flux speed, the induction motor rotates at a frequency near, but less than that of the synchronous speed.
• This slip must be present, even when operating in a field-oriented control regime. • The rotor in an induction motor is not externally excited. This means that there is no need for
slip rings and brushes. This makes the induction motor robust, inexpensive and need less maintenance.
• Torque production is governed by the angle formed between the rotor and the stator magnetic fluxes.
In Fig.2 the rotor speed is denoted by Ω . Stator and rotor frequencies are linked by a parameter called the slip s, expressed in per unit as srss ωωω /)( −= .
where s is called the “slip”: it represents the difference between the synchronous frequency and the actual motor rotating speed.
psssrad
psrad
psrad
SS
SSS
ω
ωω
)1().1()/
:)/:)/
−=Ω−=Ω
=Ω
:( speed rotating rotor -
number pairs poles stator ( freqsupply AC . :( speed field rotating stator -
Skewed Cage Bars
End Rings
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Scalar Control
Technical Background In the V/Hz control, the speed of induction motor is controlled by the adjustable magnitude of stator voltages and frequency in such a way that the air gap flux is always maintained at the desired value at the steady-state. Sometimes this scheme is called the scalar control because it focuses only on the steady-state dynamic. It can explain how this technique works by looking at the simplified version of the steady-state equivalent circuit as seen in Figure 3. According to in this figure, the stator resistance (Rs) is assumed to be zero and the stator leakage inductance (Lls) is embedded into the (referred to stator) rotor leakage inductance (Llr) and the magnetizing inductance, which is representing the amount of air gap flux, is moved in front of the total leakage inductance (Ll = Lls + Llr). As a result, the magnetizing current that generates the air gap flux can be approximately the stator voltage to frequency ratio. Its phasor equation (for steady-state analysis) can be seen as:
m
s
LV
Iωj
~~
m ≅ (1)
If the induction motor is operating in the linear magnetic region, the Lm is constant. Then, the equation (1) can be simply shown in terms of magnitude as:
fV
LfV
LI s
mm
s
m
mm ∝Λ⇒≅
Λ=
)2( π (2)
where V and Λ are their magnitude of stator voltage and stator flux, and V~ is the phasor representation, respectively.
Figure 3: Simplified steady-state equivalent circuit of induction motor From the last equation, it follows that if the ratio fV remains constant for any change in f , then flux remains constant and the torque becomes independent of the supply frequency. In order to keep mΛ constant, the ratio of fVs would also be constant at the different speed. As the speed increases, the stator voltages must, therefore, be proportionally increased in order to keep the constant ratio of Vs/f. However, the frequency (or synchronous speed) is not the real speed because of a slip as a function of the motor load. At no-load torque, the slip is very small, and the speed is nearly the synchronous speed. Thus, the simple open-loop Vs/f (or V/Hz) system cannot precisely control the speed with a presence of load torque. The slip compensation can be simply added in the system with the speed measurement. The closed-loop V/Hz system with a speed sensor can be shown in Figure 4.
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In practice, the stator voltage to frequency ratio is usually based on the rated values of these variables. The typical V/Hz profile can be shown in Figure 5. Basically, there are three speed ranges in the V/Hz profile as follows:
• At 0-fc Hz, a voltage is required, so the voltage drop across the stator resistance cannot be neglected and must be compensated for by increasing the Vs. So, the V/Hz profile is not linear. The cutoff frequency (fc) and the suitable stator voltages may be analytically computed from the steady-state equivalent circuit with Rs ≠ 0.
• At fc-frated Hz, it follows the constant V/Hz relationship. The slope actually represents the air gap flux quantity as seen in equation (2).
• At higher frated Hz, the constant Vs/f ratio cannot be satisfied because the stator voltages would be limited at the rated value in order to avoid insulation breakdown at stator windings. Therefore, the resulting air gap flux would be reduced, and this will unavoidably cause the decreasing developed torque correspondingly. This region is usually so called “fieldweakening region”. To avoid this, constant V/Hz principle is also violated at such frequencies.
Figure 4: Stator voltage versus frequency profile under V/Hz control Since the stator flux is maintained constant (independent of the change in supply frequency), the torque developed depends only on the slip speed. This is shown in Figure 5. So by regulating the slip speed, the torque and speed of an AC Induction motor can be controlled with the constant V/Hz principle.
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Slip speedω
Torq
ue
Figure 5: Torque versus slip speed of an induction motor with constant stator flux
Both open and closed-loop control of the speed of an AC induction motor can be implemented based on the constant V/Hz principle. Open-loop speed control is used when accuracy in speed response is not a concern such as in HVAC (heating, ventilation and air conditioning), fan or blower applications. In this case, the supply frequency is determined based on the desired speed and the assumption that the motor will roughly follow its synchronous speed. The error in speed resulted from slip of the motor is considered acceptable. In this implementation, the profile in Figure 4 is modified by imposing a lower limit on frequency. This is shown in Figure 6. This approach is acceptable to applications such as fan and blower drives where the speed response at low end is not critical. Since the rated voltage, which is also the maximum voltage, is applied to the motor at rated frequency, only the rated minimum and maximum frequency information is needed to implement the profile.
Frequency
Vol
tage
f ratefmin
Vra
teV
min
Figure 6: Modified V/Hz profile
The command frequency is allowed to go below the minimum frequency, fmin, with the output voltage saturating at a minimum value, Vmin . Also, when the command frequency is higher than the maximum frequency, fmax, the output voltage is saturated at a maximum value, Vmax.
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Benefits of 32-bit C2000 Controllers for Digital Motor Control (DMC) C2000 family of devices posses the desired computation power to execute complex control algorithms along with the right mix of peripherals to interface with the various components of the DMC hardware like the ADC, ePWM, QEP, eCAP etc. These peripherals have all the necessary hooks for implementing systems which meet safety requirements, like the trip zones for PWMs and comparators. Along with this the C2000 ecosystem of software (libraries and application software) and hardware (application kits) help in reducing the time and effort needed to develop a Digital Motor Control solution. The DMC Library provides configurable blocks that can be reused to implement new control strategies. IQMath Library enables easy migration from floating point algorithms to fixed point thus accelerating the development cycle. Thus, with C2000 family of devices it is easy and quick to implement complex control algorithms (sensored and sensorless) for motor control. The use of C2000 devices and advanced control schemes provides the following system improvements
• Favors system cost reduction by an efficient control in all speed range implying right dimensioning of power device circuits
• Use of advanced control algorithms it is possible to reduce torque ripple, thus resulting in lower vibration and longer life time of the motor
• Advanced control algorithms reduce harmonics generated by the inverter thus reducing filter cost.
• Use of sensorless algorithms eliminates the need for speed or position sensor.
• Decreases the number of look-up tables which reduces the amount of memory required
• The Real-time generation of smooth near-optimal reference profiles and move trajectories, results in better-performance
• Generation of high resolution PWM’s is possible with the use of ePWM peripheral for controlling the power switching inverters
• Provides single chip control system
• For advanced controls, C2000 controllers can also perform the following:
• Enables control of multi-variable and complex systems using modern intelligent methods such as neural networks and fuzzy logic.
• Performs adaptive control. C2000 controllers have the speed capabilities to concurrently monitor the system and control it. A dynamic control algorithm adapts itself in real time to variations in system behavior.
• Performs parameter identification for sensorless control algorithms, self commissioning, online parameter estimation update.
• Performs advanced torque ripple and acoustic noise reduction.
• Provides diagnostic monitoring with spectrum analysis. By observing the frequency spectrum of mechanical vibrations, failure modes can be predicted in early stages.
• Produces sharp-cut-off notch filters that eliminate narrow-band mechanical resonance. Notch filters remove energy that would otherwise excite resonant modes and possibly make the system unstable.
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TI Literature and Digital Motor Control (DMC) Library The Digital Motor Control (DMC) library is composed of functions represented as blocks. These blocks are categorized as Transforms & Estimators (Sliding Mode Observer, Phase Voltage Calculation, and Resolver, Flux, and Speed Calculators and Estimators), Control (Signal Generation, PID, BEMF Commutation, Space Vector Generation), and Peripheral Drivers (PWM abstraction for multiple topologies and techniques, ADC drivers, and motor sensor interfaces). Each block is a modular software macro is separately documented with source code, use, and technical theory. Check the folders below for the source codes and explanations of macro blocks: C:\TI\controlSUITE\libs\app_libs\motor_control\math_blocks\v4.0 C:\TI\controlSUITE\libs\app_libs\motor_control\drivers\f2803x_v2.0
These modules allow users to quickly build, or customize, their own systems. The Library supports the three motor types: ACI, BLDC, PMSM, and comprises both peripheral dependent (software drivers) and target dependent modules. The DMC Library components have been used by TI to provide system examples. At initialization all DMC Library variables are defined and inter-connected. At run-time the macro functions are called in order. Each system is built using an incremental build approach, which allows some sections of the code to be built at a time, so that the developer can verify each section of their application one step at a time. This is critical in real-time control applications where so many different variables can affect the system and many different motor parameters need to be tuned. Note: TI DMC modules are written in form of macros for optimization purposes (refer to application note SPRAAK2 for more details at TI website). The macros are defined in the header files. The user can open the respective header file and change the macro definition, if needed. In the macro definitions, there should be a backslash ”\” at the end of each line as shown below which means that the code continues in the next line. Any characters including invisible ones like “space” or “tab” after the backslash will cause compilation error. Therefore, make sure that the backslash is the last character in the line. In terms of code development, the macros are almost identical to C function, and the user can easily convert the macro definition to a C functions.
#define PARK_MACRO(v) \ \
v.Ds = _IQmpy(v.Alpha,v.Cosine) + _IQmpy(v.Beta,v.Sine); \ v.Qs = _IQmpy(v.Beta,v.Cosine) - _IQmpy(v.Alpha,v.Sine);
A typical DMC macro definition
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System Overview This section describes the “C” real-time control framework used to demonstrate the scalar control of induction motors. The “C” framework is designed to run on TMS320F2803x based controllers on Code Composer Studio. The framework uses the following modules1: Macro Names Explanation PI PI Regulator RC Ramp Controller (slew rate limiter) VHZ_PROFILE Voltage/Hertz Profile QEP / CAP QEP and CAP Drives SPEED_PR Speed Measurement (based on sensor signal period) SPEED_FR Speed Measurement (based on sensor signal frequency) SVGEN_MF Space Vector PWM (based on magnitude and frequency) PWM / PWMDAC PWM and PWMDAC Drives 1 Please refer to pdf documents in motor control folder explaining the details and theoretical background of each macro
In this system, the scalar control (V/Hz) of Induction Motor will explore the performance of speed control. The user can quickly start evaluating the performance of V/Hz system. HVACI_Scalar project has the following properties:
C Framework
System Name Program Memory Usage 2803x
Data Memory Usage1
2803x HVACI_Scalar 3602 1192
1 Excluding the stack size 2 Excluding “IQmath” Look-up Tables
* At 10 kHz ISR frequency. Optional macros excluded (i.e. PWMDAC, Datalog )
System Features Development /Emulation Code Composer Studio V4.0(or above) with Real Time debugging Target Controller TMS320F2803x PWM Frequency 10kHz PWM (Default), 60kHz PWMDAC PWM Mode Symmetrical with a programmable dead band Interrupts EPWM1 Time Base CNT_Zero – Implements 10 kHz ISR execution rate Peripherals Used
PWM 1 / 2 / 3 for motor control PWM 6A, 6B, 7A & 7B for DAC outputs QEP1 A,B, I or CAP1
CPU Utilization Total Number of Cycles 459* CPU Utilization @ 60 Mhz 7.7% CPU Utilization @ 40 Mhz 11.4%
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The overall system implementing a 3-ph induction motor V/Hz drive implementation is depicted in Figure 7. The induction motor is driven by the conventional voltage-source inverter. The TMS320F2803x is being used to generate the six pulse width modulation (PWM) signals using a space vector PWM technique, for six power switching devices in the inverter.
Figure 7: A 3-ph induction motor V/Hz drive implementation
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c_int0
Initialize S/W modules
Initialize timebases
Enable EPWM1 timebase CNT_zero interrupt and
core interrupt INT3
Initialize other system and module
parameters
Background Loop INT3
EPWM1_INT_ISR
Save contexts and clear
interrupt flags
Execute the RC module
Execute the V/Hz module
INT3 interrupt
Execute the SVGEN_MF
module
Execute the PWMGEN module
Execute the QEP/CAP module
Execute the SPEED_FR/PR module
Update the PWMDAC
Restore Contexts
Return to Background
Loop
Execute the PI module
Figure 8: System software flowchart
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Hardware Configuration (HVDMC R1.1 Kit) Please refer to the HVMotorCtrl+PFC How to Run Guide found: C:\TI\controlSUITE\development_kits\HVMotorCtrl+PfcKit_v2.0\~Docs\ for an overview of the kit’s hardware and steps on how to setup this kit. Some of the hardware setup instructions are captured below for quick reference
HW Setup Instructions
1. Open the Lid of the HV Kit
2. Install the Jumpers [Main]-J3, J4 and J5, J9 for 3.3V, 5V and 15V power rails and JTAG reset line.
3. Unpack the DIMM style controlCARD and place it in the connector slot of [Main]-J1. Push vertically down using even pressure from both ends of the card until the clips snap and lock. (to remove the card simply spread open the retaining clip with thumbs)
4. Connect a USB cable to connector [M3]-JP1. This will enable isolated JTAG emulation to the C2000 device. [M3]-LD1 should turn on. Make sure [M3]-J5 is not populated. If the included Code Composer Studio is installed, the drivers for the onboard JTAG emulation will automatically be installed. If a windows installation window appears try to automatically install drivers from those already on your computer. The emulation drivers are found at http://www.ftdichip.com/Drivers/D2XX.htm. The correct driver is the one listed to support the FT2232.
5. If a third party JTAG emulator is used, connect the JTAG header to [M3]-J2 and additionally [M3]-J5 needs to be populated to put the onboard JTAG chip in reset.
6. Ensure that [M6]-SW1 is in the “Off” position. Connect 15V DC power supply to [M6]-JP1.
7. Turn on [M6]-SW1. Now [M6]-LD1 should turn on. Notice the control card LED would light up as well indicating the control card is receiving power from the board.
8. Note that the motor should be connected to the [M5]-TB3 terminals after you finish with the first incremental build step.
9. Note the DC Bus power should only be applied during incremental build levels when instructed to do so. The two options to get DC Bus power are discussed below,
(i) To use DC power supply, set the power supply output to zero and connect [Main]-BS5 and BS6 to DC power supply and ground respectively.
(ii) To use AC Mains Power, Connect [Main]-BS1 and BS5 to each other using banana plug cord. Now connect one end of the AC power cord to [Main]-P1. The other end needs to be connected to output of a variac. Make sure that the variac output is set to zero and it is connected to the wall supply through an isolator.
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For reference the pictures below show the jumper and connectors that need to be connected for this lab.
CAUTION: The inverter bus capacitors remain charged for a long time after the high power line supply is switched off/disconnected. Proceed with caution!
ACI Motor
Encoder or Tacho
15V DC
AC Entry
J3,J4,J5 J9
Fig. 12 Using AC Power to generate DC Bus Power
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CAUTION: The inverter bus capacitors remain charged for a long time after the high power line supply is switched off/disconnected. Proceed with caution!
ACI Motor
Encoder or Tacho
15V DC
J3,J4,J5 J9
DC Power Supply (max. 350V) + -
Fig. 13 Using External DC power supply to generate DC-Bus for the inverter
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Software Setup Instructions to Run HVACI_Scalar Project
Please refer to the “Software Setup for HVMotorCtrl+PFC Kit Projects” section in the HVMotorCtrl+PFC Kit How to Run Guide which can be found at C:\TI\controlSUITE\development_kits\HVMotorCtrl+PfcKit_v2.0\~Docs This section goes over how to install CCS and set it up to run with this project. Select the HVACI_Scalar as the active project. Select the active build configuration to be set as F2803x_RAM. Verify that the build level is set to 1, and then right click on the project name and select “Rebuild Project”. Once build completes, launch a debug session to load the code into the controller. Now open a watch window and add the critical variables as shown in the table below and select the appropriate Q format for them.
Table 1: Watch Window Variables
Setup time graph windows by importing Graph1.graphProp and Graph2.graphProp from the following location :\TI\ControlSUITE\developement_kits\HVMotorCtrl+PfcKit_v2.0\HVACI_Scalar. To do this, go to Tools Graph Dual Time, and click the Import button at the bottom of the window. Click on Continuous Refresh button on the top left corner of the graph tab to enable periodic capture of data from the microcontroller.
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Incremental System Build
The system is gradually built up so the final system can be confidently operated. Three phases of the incremental system build are designed to verify the major software modules used in the system. Table 2 summarizes the modules testing and using in each incremental system build.
Software Module Phase 1 Phase 2 Phase 3 VHZ_PROF_MACRO √√ (1a) √ √ SVGEN_MF_MACRO √√ (1a) √ √ PWMDAC_MACRO √√ (1b) √ √ PWM_MACRO √√ (1c) √ √ RC_MACRO √ √ QEP_MACRO √√ (2a) √ SPEED_FR_MACRO √√ (2a) √ CAP_MACRO √√ (2b) √ SPEED_PR_MACRO √√ (2b) √ PI_MACRO (SPD) √√ Note: the symbol √ means this module is using and the symbol √√ means this module is testing in this phase.
Table 2: Testing modules in each incremental system build
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Level 1 Incremental Build
At this step keep the motor disconnected. Assuming the load and build steps described in the “HVMotorCtrl+PFC Kit How To Run Guide” completed successfully, this section describes the steps for a “minimum” system check-out which confirms operation of system interrupt, the peripheral & target independent VHZ_PROF_MACRO (V/Hz Linearity) and SVGENMF_MACRO (space vector generator) modules. Open HVACI_Scalar-Settings.h and select level 1 incremental build option by setting the BUILDLEVEL to LEVEL1 (#define BUILDLEVEL LEVEL1). Now right click on the project name and click Rebuild Project. Once the build is complete click on debug button, reset CPU, restart, enable real time mode and run. Set “EnableFlag” to 1 in the watch window. The variable named “IsrTicker” will now keep on increasing, confirm this by watching the variable in the watch window. This confirms that the system interrupt is working properly. In the software, the key variables to be adjusted are summarized below. • SpeedRef (Q24): for changing the rotor speed in per-unit Level 1A – V/Hz and SVGEN_MF Macro Testing
Before testing this phase, the knowledge of V/Hz profile has to be kept in mind. The base frequency may be specified at 2 times of rated frequency (60 Hz) in order to run the motor at speed higher than rated speed (in the field weakening). Figure 11 shows an example of V/Hz profile used in this system.
Figure 11: V/Hz profile configuration used in this system
The SpeedRef value is specified to the VHZ_PROF_MACRO module. The VHZ_PROF_MACRO module is generating the outputs to the SVGENMF_MACRO module. Three outputs from SVGENMF_MACRO module are monitored via the graph window as shown in Figure 12 where Ta, Tb, and Tc waveforms are 120° apart from each other. Specifically, Tb lags Ta by 120° and Tc leads Ta by 120°. During running this build, the waveforms in the CCS graphs should appear as follows:
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Level 1B – PWMDAC Macro Testing
To monitor internal signal values in real time PWM DACs are very useful tools. Present on the HV DMC board are PWM DAC’s which use an external low pass filter to generate the waveforms ([Main]-J14, DAC-1 to 4). A simple 1st–order low-pass filter RC circuit is used to filter out the high frequency components. The selection of R and C value (or the time constant, τ) is based on the cut-off frequency (fc), for this type of filter; the relation is as follows:
cfRC
πτ
21
==
For example, R=1.8kΩ and C=100nF, it gives fc = 884.2 Hz. This cut-off frequency has to be below the PWM frequency. Using the formula above, one can customize low pass filters used for signal being monitored.
The DAC circuit low pass filters ([Main]-R10 to13 & [Main]-C15 to18) is shipped with 2.2kΩ and 220nF on the board. Refer to application note SPRAA88A for more details at TI website.
Figure 13: DAC-1-4 outputs showing Ta, Tb, Tc and Ta-Tb waveforms
Figure 12: SVGEN duty cycle outputs Ta, Tb, Tc and Ta-Tb
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Level 1C – PWM_MACRO and INVERTER Testing
After verifying SVGENMF_MACRO module in Level 1A, the PWM_MACRO software module and the 3-phase inverter hardware are tested by looking at the low pass filter outputs. For this purpose, if using the external DC power supply, gradually increase the DC bus voltage and check the Vfb-U, V and W test points using an oscilloscope or if using AC power entry slowly change the variac to generate the DC bus voltage. Once the DC Bus voltage is greater than 15 to 20V you would start observing the Inverter phase voltage dividers and waveform monitoring filters (Vfb-U, Vfb-V, Vfb-W) enable the generation of the waveform and ensures theta the inverter is working appropriately. Note that the default RC values are optimized for AC motor state observers employing phase voltages. Once the instructions given in Level1A to 1C are successfully completed and the system is shut down as suggested below, the motor can be connected. Now the system is ready to run open loop.
After verifying this, reduce the DC Bus voltage, take the controller out of real time mode (disable), reset the processor (see “HVMotorCtrl+PFC Kit How To Run Guide” for details). Note that after each test, this step needs to be repeated for safety purposes.
Also note that improper shutdown might halt the PWMs at some certain states where high currents can be drawn, hence caution needs to be taken while doing these experiments.
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VHZ PROFMACRO
SVGEN_MFMACRO
PWM MACRO
HW
PWM1 A/B
PWM2 A/B
PWM3 A/B
Mfunc_C1
Mfunc_C3
Mfunc_C2
Ta
Tc
Tb
Freq
Gain
Offset
VoltOutFreqSpeedRef
PWMDACMACRO MFuncC1
MFuncC2
PWMxA
PWMxB
Low Pass Filter Cct
DATALOG
Dlog1
Dlog2
Dlog3
Dlog4
Level 1 - Incremental System Build Block Diagram
Level 1 verifies the target independent modules V/Hz and SVGENMF
Scope Graph Window
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Level 2 Incremental Build Assuming section Level 1 is completed successfully, this section verifies the speed feedback drives and macros. In this phase, the induction motor is connected. The open-loop V/Hz control of induction motor is performed. Open HVACI_Scalar-Settings.h and select level 2 incremental build option by setting the BUILDLEVEL to LEVEL2 (#define BUILDLEVEL LEVEL2) and save the file. Now Right Click on the project name and click Rebuild Project. Once the build is complete click on debug button, reset CPU, restart, enable real time mode and run. Set “EnableFlag” to 1 in the watch window. The variable named “IsrTicker” will be incrementally increased as seen in watch windows to confirm the interrupt working properly. In the software, the key variables to be adjusted are summarized below. SpeedRef (Q24): for changing the rotor speed in per-unit. LEVEL 2A – Testing the QEP and SPEED_FR (for incremental encoder) This section verifies the QEP1 driver and its speed calculation. QEP drive macro determines the rotor position and generates a direction (of rotation) signal from the shaft position encoder pulses. Make sure that the output of the incremental encoder is connected to [Main]-H1 and QEP/SPEED_FR macros are initialized properly in the HVACI_Scalar.c file depending on the features of the speed sensor. Refer to the pdf files regarding the details of related macros in motor control folder (C:\TI\controlSUITE\libs\app_libs\motor_control). The steps to verify these two software modules related to the speed measurement can be described as follows:
• Set SpeedRef to 0.4 pu (or another suitable value if the base speed is different).
• Compile/load/run program with real time mode and then increase voltage at variac / dc power supply to get the appropriate DC-bus voltage. Now the motor is running close to reference speed.
• Check the “speed1.Speed” in the watch windows with continuous refresh feature whether or not the measured speed is less than SpeedRef a little bit due to a slip of motor.
• To confirm these modules, try different values of SpeedRef to test the Speed.
• Use oscilloscope to view the electrical angle output, ElecTheta, from QEP_MACRO module and the emulated rotor angle, Out, from RG_MACRO at PWMDAC outputs with external low-pass filters.
• Check that both ElecTheta and Out are of saw-tooth wave shape and have the same period. If the measured angle is in opposite direction, then change the order of motor cables connected to inverter output (TB3 for HVDMC kit).
• Check from Watch Window that qep1.IndexSyncFlag is set back to 0xF0 every time it reset to 0 by hand. Add the variable to the watch window if it is not already in the watch window.
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Level 2B – Testing the CAP and SPEED_PR (for tacho or sprocket) In this case, the CAP1 input is chosen to detect the edge. If available, make sure that the sensor output is connected to [Main]-H1 and CAP/SPEED_PR macros are initialized properly in the HVACI_Scalar.c file depending on the features of the speed sensor. Typically the capture is used to measure speed when a simple low cost speed sensing system is available. The sensor generates pulses when detecting the teeth of a sprocket or gear and the capture drive provides the instantaneous value of the selected time base (GP Timer) captured on the occurrence of an event. Refer to the pdf files regarding the details of related macros in motor control folder (C:\TI\controlSUITE\libs\app_libs\motor_control). The steps to verify these two software modules related to the speed measurement can be described as follows: • Set SpeedRef to 0.4 pu (or suitable value if the base speed is different).
• Compile/load/run program with real time mode and then increase voltage at variac to get the
appropriate DC-bus voltage. Now the motor is running with this synchronous speed (0.4 pu).
• Check the “speed2.Speed” in the watch windows with continuous refresh feature whether or not they should be less than SpeedRef a little bit due to a slip of motor.
• To confirm these modules, try different values of SpeedRef to test the Speed.
• Reduce voltage at variac / dc power supply to zero, halt program and stop real time mode. Now the motor is stopping
An alternative to verify these two software modules without running the motor can be done by using a function generator. The key steps can be explained as follows:
• Use a function generator to generate the 3.3V (DC) square-wave with the desired frequency corresponding to the number of teeth in sprocket and the wanted speed in rpm. Then, connect only the pulse signal and ground wires from the function generator to HVDMC board. The desired frequency of the square-wave produced by function generator can be formulated as:
HzTEETHRPMf wavesquare 60_×
=
where RPM is the wanted speed in rpm, and TEETH is the number of teeth in sprocket.
• Compile/load/run program with real time mode and then increase voltage at variac to get the appropriate DC-bus voltage. Now the motor is running. Note that the SpeedRef could be set to any number.
• Check the speed2.Speed and speed2.SpeedRpm in the watch windows with continuous refresh feature whether or not they should be corresponding to the wanted speed that is chosen before.
• To confirm these modules, change different frequencies of square-wave produced by function generator with corresponding wanted (known) speed to check the Speed and SpeedRpm.
Bring the system to a safe stop as described at the end of build 1 by reducing the bus voltage, taking the controller out of real-time mode and reset.
24
VHZ PROFMACRO
SVGEN_MFMACRO
PWM1 A/B
PWM2 A/B
PWM3 A/B
Mfunc_C1
Mfunc_C3
Mfunc_C2
Ta
Tc
Tb
Freq
Gain
Offset
VoltOutFreqSpeedRef
PWMDACMACRO MFuncC1
MFuncC2
PWMxA
PWMxB
Low Pass Filter Cct
DATALOG
Dlog1
Dlog2
Dlog3
Dlog4
Level 2 - Incremental System Build Block Diagram
Level 2 verifies the PWM generation, analog-to-digital conversions, and capture units
ACIMotor
RCMACRO
SetPointValue
PWM MACRO
PWMHW
CAP MACRO
CAPHW
3-Phase Inverter
QEPn
CAPn
SPEED FRMACRO ElecTheta
DirectionQEP
MACROQEPHW
ElecTheta
SPEED PRMACRO
Speed
SpeedRpm
Speed
SpeedRpm
Scope
Graph Window
25
Level 3 Incremental Build Assuming the previous section is completed successfully, this section verifies the PI module. In this phase, induction motor is connected. The speed closed-loop V/Hz control of induction motor is performed. Open HVACI_Scalar-Settings.h and select level 3 incremental build option by setting the BUILDLEVEL to LEVEL3 (#define BUILDLEVEL LEVEL3). Now Right Click on the project name and click Rebuild Project. Once the build is complete click on debug button, reset CPU, restart, enable real time mode and run. Set “EnableFlag” to 1 in the watch window. The variable named “IsrTicker” will be incrementally increased as seen in watch windows to confirm the interrupt working properly. In the software, the key variables to be adjusted are summarized below. • SpeedRef (Q24): for changing the rotor speed in per-unit. • Set ClosedLoopFlag: for switching between open-loop and closed-loop control In this build, the motor is supplied by AC input voltage and the (AC) motor current is dynamically regulated by using PI module. The steps are explained as follows:
• Set SpeedRef to 0.4 pu (or another suitable value if the base speed is different).
• Set ClosedLoop Flag to 0.
• Compile/load/run program with real time mode and then set EnableFlag to 1. Then, increase voltage at variac to get the appropriate DC-bus voltage. Now the motor is running with this set speed (0.4 pu).
• Adjust gains Kp, Ki such that the PI output is properly representing the slip.
• Then, set ClosedLoopFlag to 1.
• To confirm the performance the system, try different values of SpeedRef and check speed. Set ClosedLoopFlag to 0 immediately if the close-loop system is not successfully operated.
• Reduce voltage at variac to zero, halt program and stop real time mode. Now the motor is stopping.
26
During running this build, the waveforms in the CCS graphs should appear as follows:
Figure 14: Ta (top-left), Tb (bottom-left), speed reference (top-right), speed feedback (bottom-right)
27
VHZ PROFMACRO
SVGEN_MFMACRO
PWM1 A/B
PWM2 A/B
PWM3 A/B
Mfunc_C1
Mfunc_C3
Mfunc_C2
Ta
Tc
Tb
Freq
Gain
Offset
VoltOutFreq
SpeedRef
PWMDACMACRO MFuncC1
MFuncC2
PWMxA
PWMxB
Low Pass Filter Cct
DATALOG
Dlog1
Dlog2
Dlog3
Dlog4
Level 3 - Incremental System Build Block Diagram
Level 3 verifies PI Regulator and Closed Loop functionality
ACIMotor
RCMACRO
SetPointValue
PWM MACRO
PWMHW
CAP MACRO
CAPHW
3-Phase Inverter
QEPn
CAPn
SPEED FRMACRO ElecTheta
DirectionQEP
MACROQEPHW
ElecTheta
SPEED PRMACRO
Speed
SpeedRpm
Speed
SpeedRpm
Ref
Fbk
Out
∑ClosedLoopFlag=1
Graph Window
Scope
PI MACROIdc Reg.
